Acrylic and para-aramid pulp and processes of making same

ABSTRACT

The present invention relates to acrylic and para-aramid pulp for use as reinforcement material in products such as seals and friction materials. The pulp comprises (a) irregularly shaped, acrylic fibrous structures, (b) irregularly shaped, para-aramid fibrous structures, and (c) water, whereby acrylic fibrils and/or stalks are substantially entangled with para-aramid fibrils and/or stalks. The invention further relates to processes for making such acrylic and aramid pulp.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to acrylic and para-aramid pulp for use asreinforcement material in products, such as seals and frictionmaterials. The invention further relates to processes for making suchpulp.

2. Description of Related Art

Fibrous and non fibrous reinforcement materials have been used for manyyears in friction products, sealing products and other plastic or rubberproducts. Such reinforcement materials typically must exhibit high wearand heat resistance.

Asbestos fibers have historically been used as reinforcement materials,but due to their health risks, replacements have been made or proposed.However, many of these replacements do not perform as well as asbestosin one way or another.

Research Disclosure 74-75, published February 1980, discloses themanufacture of pulp made from fibrillated KEVLAR® brand para-aramidfibers of variable lengths and use of such pulp as a reinforcementmaterial in various applications. This publication discloses that pulpmade from KEVLAR® brand para-aramid fibers can be used in sheet productsalone, or in combination with fibers of other materials, such as NOMEX®brand meta-aramid, wood pulp, cotton and other natural cellulosics,rayon, polyester, polyolefin, nylon, polytetrafluoroethylene, asbestosand other minerals, fiberglass and other, ceramics, steel and othermetals, and carbon. The publication also discloses the use of pulp fromKEVLAR® brand para-aramid fiber alone, or with KEVLAR® brand para-aramidshort staple, in friction materials to replace a fraction of theasbestos volume, with the remainder of the asbestos volume beingreplaced by fillers or other fibers.

U.S. Pat. No. 5,811,042 (to Hoiness) discloses a composite friction orgasketing material made of a thermoset or thermoplastic matrix resin,fiber reinforcing material, and substantially fibril free aramidparticles. Poly (p-phenylene terephthalamide) and poly(m-phenyleneisophthalamide) are preferred fiber reinforcing materials, and thefibers can be in the form of floc or pulp.

U.S. Patent Application 2003/0022961 (to Kusaka et al.) disclosesfriction materials made from a friction modifier, a binder and a fibrousreinforcement made of a mixture of (a) a dry aramid pulp and (b) wetaramid pulp, wood pulp or acrylic pulp. Dry aramid pulp is defined as anaramid pulp obtained by “the dry fibrillation method”. The dryfibrillation method is dry milling the aramid fibers between a rotarycutter and a screen to prepare the pulp. Wet aramid pulp is defined asan aramid pulp obtained by “the wet fibrillation method”. The wetfibrillation method is milling short aramid fibers in water between tworotary discs to form fibrillated fibers and then dehydrating thefibrillated fibers, i.e., the pulp. Kusaka et al further disclose amethod of mix-fibrillating fibers by first mixing plural types oforganic fibers that fibrillate at a definite ratio, and thenfibrillating the mixture to produce a pulp.

There is an ongoing need to provide alternative reinforcing materialsthat both perform well in products, such as seals and frictionapplications, and that are low in cost. Despite the numerous disclosuresproposing lower cost alternative reinforcement materials, many of theseproposed products do not adequately perform in use, cost significantlymore than currently commercial products, or have other negativeattributes. As such, there remains a need for reinforcement materialsthat exhibit high wear and heat resistance, and that are comparable orless expensive than other commercially available reinforcementmaterials.

BRIEF SUMMARY OF THE INVENTION

The invention relates to a first embodiment of a process for making anacrylic and para-aramid pulp for use as reinforcement material,comprising:

-   -   (a) combining pulp ingredients including:        -   (1) acrylic fiber comprising acrylonitrile units which are            at least 85 wt % of the total acrylic fiber, the fiber being            10 to 90 wt % of the total solids in the ingredients, and            having an average length of no more than 10 cm;        -   (2) para-aramid fiber being 10 to 90 wt % of the total            solids in the ingredients, and having an average length of            no more than 10 cm; and        -   (3) water being 95 to 99 wt % of the total ingredients;    -   (b) mixing the ingredients to a substantially uniform slurry;    -   (c) co-refining the slurry by simultaneously:        -   (1) fibrillating, cutting and masticating the acrylic fiber            and the para-aramid fiber to irregularly shaped fibrillated            fibrous structures with stalks and fibrils; and        -   (2) substantially uniformly dispersing all solids in the            refined slurry; and    -   (d) removing water from the refined slurry to no more than 60        total wt % water,    -   thereby producing an acrylic and para-aramid pulp with the        acrylic and the para-aramid fibrous structures having an average        maximum dimension of no more than 5 mm, a length-weighted        average of no more than 1.3 mm, and the acrylic fibrils and/or        stalks are substantially entangled with the para-aramid fibrils        and/or stalks.

The invention is further related to a second embodiment of a process formaking an acrylic and para-aramid pulp for use as reinforcementmaterial, comprising:

-   -   (a) combining ingredients including water and a first fiber from        the group consisting of:        -   (1) acrylic fiber comprising acrylonitrile units which are            at least 85 wt % of the total acrylic fiber, the fiber being            10 to 90 wt % of the total solids in the ingredients; and        -   (2) para-aramid fiber being 10 to 90 wt % of the total            solids in the ingredients;    -   (b) mixing the ingredients to a substantially uniform        suspension;    -   (c) refining the suspension in a disc refiner thereby cutting        the fiber to have an average length of no more than 10 cm, and        fibrillating and masticating at least some of the fiber to        irregularly shaped fibrillated fibrous structures;    -   (d) combining ingredients including the refined suspension, the        second fiber of the group of (a)(1 and 2), and water, if        necessary, to increase the water concentration to 95-99 wt % of        the total ingredients;    -   (e) mixing the ingredients, if necessary, to form a        substantially uniform suspension;    -   (d) co-refining the mixed suspension by:        -   (1) fibrillating, cutting and masticating solids in the            suspension such that all or substantially all of the acrylic            and para-aramid fiber is converted to irregularly shaped            fibrillated acrylic and para-aramid fibrous structures with            stalks and fibrils; and        -   (2) substantially uniformly dispersing all solids in the            refined slurry; and    -   (h) removing water from the refined slurry to no more than 60        total wt % water,    -   thereby producing an acrylic and para-aramid pulp with the        acrylic and the para-aramid fibrous structures having an average        maximum dimension of no more than 5 mm, a length-weighted        average of no more than 1.3 mm, and the acrylic fibrils and/or        stalks are substantially entangled with the para-aramid fibrils        and/or stalks.

The invention is further directed to an acrylic and para-aramid pulp foruse as reinforcement material, comprising:

-   -   (a) irregularly shaped, acrylic fibrous structures comprising        acrylonitrile units which are at least 85 wt % of the total        acrylic fibrous structures and being 10 to 90 wt % of the total        solids;    -   (b) irregularly shaped, para-aramid fibrous structures being 10        to 90 wt % of the total solids; and    -   (c) water being 4 to 60 wt % of the entire pulp,    -   whereby the acrylic and the para-aramid fibrous structures        having an average maximum dimension of no more than 5 mm, a        length-weighted average of no more than 1.3 mm, and stalks and        fibrils where the acrylic fibrils and/or stalks are        substantially entangled with the para-aramid fibrils and/or        stalks.

The invention is further directed to a friction material, comprising afriction modifier; optionally at least one filler; a binder; and afibrous reinforcement material comprising the pulp of the presentinvention.

Moreover, the invention is directed to a sealing material, comprising abinder; optionally at least one filler; and a fibrous reinforcementmaterial comprising the pulp of the present invention.

BRIEF DESCRIPTION OF THE DRAWING(S)

The invention can be more fully understood from the following detaileddescription thereof in connection with accompanying drawings describedas follows.

FIG. 1 is a block diagram of apparatus for performing a wet process formaking “wet” pulp in accordance with the present invention.

FIG. 2 is a block diagram of apparatus for performing a dry process formaking “dry” pulp in accordance with the present invention.

FIG. 3 is an image of a photomicrograph of para-aramid particles used asan optional ingredient to the process of the present invention.

FIG. 4 is an image of a photomicrograph of pulp made according to theprocess of the present invention.

GLOSSARY

Before the invention is described, it is useful to define certain termsin the following glossary that will have the same meaning throughoutthis disclosure unless otherwise indicated.

“Fiber” means a relatively flexible, unit of matter having a high ratioof length to width across its cross-sectional area perpendicular to itslength. Herein, the term “fiber” is used interchangeably with the term“filament” or “end”. The cross section of the filaments described hereincan be any shape, but are typically circular or bean shaped. Fiber spunonto a bobbin in a package is referred to as continuous fiber. Fiber canbe cut into short lengths called staple fiber. Fiber can be cut intoeven smaller lengths called floc. Yarns, multifilament yarns or towscomprise a plurality of fibers. Yarn can be intertwined and/or twisted.

“Fibril” means a small fiber having a diameter as small as a fraction ofa micrometer to a few micrometers and having a length of from about 10to 100 micrometers. Fibrils generally extend from the main trunk of alarger fiber having a diameter of from 4 to 50 micrometers. Fibrils actas hooks or fasteners to ensnare and capture adjacent material. Somefibers fibrillate, but others do not or do not effectively fibrillateand for purposes of this definition such fibers do not fibrillate.Poly(para-phenylene terephthalamide) fiber fibrillates readily uponabrasion, creating fibrils. Acrylic fibers of this invention alsofibrillate.

“Fibrillated fibrous structures” means particles of material having astalk and fibrils extending therefrom wherein the stalk is generallycolumnar and about 10 to 50 microns in diameter and the fibrils arehair-like members only a fraction of a micron or a few microns indiameter attached to the stalk and about 10 to 100 microns long.

“Floc” means short lengths of fiber, shorter than staple fiber. Thelength of floc is about 0.5 to about 15 mm and a diameter of 4 to 50micrometers, preferably having a length of 1 to 12 mm and a diameter of8 to 40 micrometers. Floc that is less than about 1 mm does not addsignificantly to the strength of the material in which it is used. Flocor fiber that is more than about 15 mm often does not function wellbecause the individual fibers may become entangled and cannot beadequately and uniformly distributed throughout the material or slurry.Aramid floc is made by cutting aramid fibers into short lengths withoutsignificant or any fibrillation, such as those prepared by processesdescribed in U.S. Pat. Nos. 3,063,966, 3,133,138, 3,767,756, and3,869,430.

“Length-weighted average” means the calculated length from the followingformula:$\text{Length-weighted~~average} = \frac{\sum\left\lbrack \left( \text{Each~~individual~~pulp~~length} \right)^{2} \right\rbrack}{\sum\left\lbrack \text{Each~~individual~~pulp~~length} \right\rbrack}$

“Maximum dimension” of an object means the straight distance between thetwo most distal points from one another in the object

“Staple fiber” can be made by cutting filaments into lengths of no morethan 15 cm, preferably 3 to 15 cm; and most preferably 3 to 8 cm. Thestaple fiber can be straight (i.e., non crimped) or crimped to have asaw tooth shaped crimp along its length, with any crimp (or repeatingbend) frequency. The fibers can be present in uncoated, or coated, orotherwise pretreated (for example, pre-stretched or heat-treated) form.

DETAILED DESCRIPTION OF THE INVENTION

The invention is directed to processes for making an acrylic andpara-aramid pulp for use as reinforcement material. The invention isalso directed to acrylic and para-aramid pulp, that can be made by theprocesses of the invention, for use as reinforcement material. Theinvention is further directed to products, such as sealing materials andfriction materials, incorporating the pulp of this invention, andprocesses for making them.

I. First Embodiment of the Inventive Process

In a first embodiment, the process for making an acrylic and para-aramidpulp comprises the following steps. First, pulp ingredients arecombined, added or contacted together. Second, the combined pulpingredients are mixed to a substantially uniform slurry. Third, theslurry is simultaneously refined or co-refined. Fourth, water is removedfrom the refined slurry.

Combining Step

In the combining step, the pulp ingredients are preferably addedtogether in a container. The pulp ingredients include (1) acrylic fiber,(2) para-aramid fiber, (3) optionally substantially or completelyfibril-free, granular, para-aramid particles, (4) optionally other minoradditives, and (5) water.

Acrylic Fiber

The acrylic fiber is added to a concentration of 10 to 90 wt % of thetotal solids in the ingredients, preferably 25 to 60 wt % of the totalsolids in the ingredients, and most preferably 25 to 55 wt % of thetotal solids in the ingredients.

The acrylic fiber preferably has an average length of no more than 10cm, more preferably 0.5 to 5 cm, and most preferably 0.6 to 2 cm. Priorto combining the pulp ingredients together, any acrylic fibers in theform of continuous filaments can be cut into shorter fibers, such asstaple fibers or floc.

Acrylic Polymer

The acrylic fiber useful in this invention includes acrylonitrile unitswhich are at least 85 wt % of the total acrylic fiber. An acrylonitrileunit is —(CH2-CHCN)—. The acrylic fiber can be made from acrylicpolymers made up of 85% by weight or more of acrylonitrile with 15% byweight or less of an ethylenic monomer copolymerizable withacrylonitrile and mixtures of two or more of these acrylic polymers.Examples of the ethylenic monomer copolymerizable with acylonitrileinclude acylic acid, methacrylic acid and esters thereof (methylacrylate, ethyl acrylate, methyl methacylate, ethyl methacrylate, etc.),vinyl acetate, vinyl chloride, vinylidene chloride, acrylamide,methacylamide, methacrylonitrile, allylsulfonic acid, methanesulfonicacid and styrenesulfonic acid.

Para-Aramid Fiber

The para-aramid fiber is added to a concentration of 10 to 90 wt % ofthe total solids in the ingredients, preferably 40 to 75 wt % of thetotal solids in the ingredients, and most preferably 40 to 55 wt % ofthe total solids in the ingredients. The para-aramid fiber preferablyhas a linear density of no more than 10 dtex, more preferably 0.5 to 10dtex, and most preferably, 0.8 to 2.5 dtex. The para-aramid fiber alsopreferably has an average length along its longitudinal axis of no morethan 10 cm, more preferably an average length of 0.65 to 2.5 cm, andmost preferably an average length of 0.65 to 1.25 cm.

Para-Aramid Particles

Optionally, in one embodiment, the pulp ingredients further includesubstantially or completely fibril-free, granular, para-aramidparticles. If these particles are added, they are added to aconcentration of no more than 50 wt % of the total solids in theingredients, preferably 20 to 50 wt % of the total solids in theingredients, and most preferably 25 to 35 wt % of the total solids inthe ingredients. Being made of para-aramid, they contribute superiorwear resistance and dispersability to the pulp being produced. Becausethe particles are substantially fibril-free, they also serve as acompounding agent to assist in dispersing the other ingredients in themixture and slurry. Particles that perform this function are often knownas processing agents or aids. The substantially or completelyfibril-free, granular, para-aramid particles have an average maximumdimension of 50 to 2000 microns (0.05 to 2 mm), preferably 50 to 1500microns, and most preferably 75 to 1000 microns. Particles below about50 microns, however, lose effectiveness in friction and sealingapplications. Particles above about 2000 microns do not adequately staydispersed in the water with the other ingredients when mixed. FIG. 5 isan image of a photomicrograph of para-aramid particles capable of beingused as an ingredient to the process of the present invention.

Aramid Polymer

Polymers suitable for use in making the aramid fiber and aramidparticles of this invention are synthetic aromatic polyamides. Thepolymers must be of fiber-forming molecular weight in order to be shapedinto fibers. The polymers can include polyamide homopolymers,copolymers, and mixtures thereof which are predominantly aromatic,wherein at least 85% of the amide (—CONH—) linkages are attacheddirectly to two aromatic rings. The rings can be unsubstituted orsubstituted. The polymers are para-aramid when the two rings are paraoriented with respect to each other along the molecular chain.Preferably copolymers have no more than 10 percent of other diaminessubstituted for a primary diamine used in forming the polymer or no morethan 10 percent of other diacid chlorides substituted for a primarydiacid chloride used in forming the polymer. Additives can be used withthe aramid; and it has been found that up to as much as 13 percent byweight of other polymeric material can be blended or bonded with thearamid. The preferred para-aramids are poly(para-phenyleneterephthalamide)(PPD-T) and its copolymers.

Optional Other Additives

Other additives can optionally be added as long as they stay suspendedin solution in the mixing step and do not significantly change theeffect of the refining step on the mandatory solid ingredients listedabove. Suitable additives include pigments, dyes, anti-oxidants,flame-retardant compounds, and other processing and dispersing aids.Preferably, the pulp ingredients do not include asbestos. In otherwords, the resulting pulp is asbestos free or without asbestos.

Water

Water is added to a concentration of 95 to 99 wt % of the totalingredients, and preferably 97 to 99 wt % of the total ingredients.Further, the water can be added first. Then other ingredients can beadded at a rate to optimize dispersion in the water while simultaneouslymixing the combined ingredients.

Mixing Step

In the mixing step, the ingredients are mixed to a substantially uniformslurry. By “substantially uniform” is meant that random samples of theslurry contain the same wt % of the concentration of each of thestarting ingredients as in the total ingredients in the combination stepplus or minus 10 wt %, preferably 5 wt % and most preferably 2 wt %. Forinstance, if the concentration of the solids in the total mixture is 50wt % acrylic fiber plus 50 wt % para-aramid fiber, then a substantiallyuniform mixture in the mixing step means each random sample of theslurry has (1) a concentration of the acrylic fiber of 50 wt % plus orminus 10 wt %, preferably 5 wt % and most preferably 2 wt % and (2) aconcentration of para-aramid fiber of 50 wt % plus or minus 10 wt %,preferably 5 wt % and most preferably 2 wt %. The mixing can beaccomplished in any vessel containing rotating blades or some otheragitator. The mixing can occur after the ingredients are added or whilethe ingredients are being added or combined.

Refining Step

In the refining step the pulp ingredients are simultaneously co-refined,converted or modified as follows. The acrylic fiber and the para-aramidfiber are fibrillated, cut and masticated to irregularly shaped fibrousstructures having stalks and fibrils. If para-aramid particles are addedwith the other ingredients, at least some of the para-aramid particlesare masticated into smaller, rounder, substantially fibril-free,particles. All solids are dispersed such that the refined slurry issubstantially uniform. “Substantially uniform” is as defined above. Therefining step preferably comprises passing the mixed slurry through oneor more disc refiner, or recycling the slurry back through a singlerefiner. By the term “disc refiner” is meant a refiner containing one ormore pair of discs that rotate with respect to each other therebyrefining ingredients by the shear action between the discs. In onesuitable type of disc refiner, the slurry being refined is pumpedbetween closely spaced circular rotor and stator discs rotatable withrespect to one another. Each disc has a surface, facing the other disc,with at least partially radially extending surface grooves. A preferreddisc refiner that can be used is disclosed in U.S. Pat. No. 4,472,241.If necessary for uniform dispersion and adequate refining, the mixedslurry can be passed through the disc refiner more than once or througha series of at least two disc refiners. When the mixed slurry is refinedin only one refiner, there is a tendency for the resulting slurry to beinadequately refined and non uniformly dispersed. Conglomerates oraggregates entirely or substantially of one solid ingredient, or theother, or both, or all three if three are present, can form rather thanbeing dispersed forming a substantially uniform dispersion. Suchconglomerates or aggregates have a greater tendency to be broken apartand dispersed in the slurry when the mixed slurry is passed through therefiner more than once or passed through more than one refiner.

Because a substantially uniform slurry containing multiple ingredientsis co-refined in this step of the process, any one type of non-pulpingredient (for example, para-aramid fiber) is refined into a pulp inthe presence of all the other types of non-pulp ingredients (forexample, aramid material pieces and optionally para-aramid particles)while those other ingredients are also being refined. This co-refiningof non-pulp ingredients forms a pulp that is superior to a pulp blendgenerated by merely mixing two pulps together. Adding two pulps and thenmerely mixing them together does not form the substantially uniform,intimately connected, fibrous components of the pulp generated byco-refining of non-pulp ingredients into pulp in accordance with thepresent invention.

Removing Step

Then water is removed from the refined slurry to no more than 60 totalwt % water, preferably 4 to 60 total wt % water, most preferably, 5 to58 total wt % water. The water can be removed by collecting the pulp ona dewatering device such as a horizontal filter, and if desired,additional water can be removed by applying pressure or squeezing thepulp filter cake. The dewatered pulp can optionally then be dried to adesired moisture content, and/or can be packaged or wound up on rolls.

FIGS. 1 and 2

This process will now be described in reference to FIGS. 1 and 2.Throughout this detailed description, similar reference characters referto similar elements in all figures of the drawings.

Referring to FIG. 1, there is a block diagram of an embodiment of a wetprocess for making “wet” pulp in accordance with the present invention.Pulp ingredients 1 are added to container 2. Container 2 is providedwith an internal mixer, similar to a mixer in a washing machine. Themixer disperses the ingredients into the water creating thesubstantially uniform slurry. The mixed slurry is transferred to a firstrefiner 3 which refines the slurry. Then, optionally, the refined slurrycan be transferred to a second refiner 4, and optionally then to a thirdrefiner 5. Three refiners are illustrated but any number of refiners canbe used depending on the degree of uniformity and refining desired.After the last refiner in the series of refiners, the refined slurry isoptionally transferred to a filter or sorter 6 that allows slurry withdispersed solids below a chosen mesh or screen size to pass andrecirculates dispersed solids larger than a chosen mesh or screen sizeback to one or more of the refiners such as through line 7 or to arefiner 8 dedicated to refine this recirculated slurry from whichrefined slurry is again passed to the filter or sorter 6. Suitablyrefined slurry passes from the filter or sorter 6 to a horizontal watervacuum filter 9 which removes water such that the pulp has aconcentration of water of no more than 75 wt % of the total ingredients.Slurry can be transferred from point to point by any conventional methodand apparatus such as with the assistance of one or more pump 10. Thenthe pulp is conveyed to a dryer 11 that removes more water until thepulp has a concentration of water of no more than 60 wt % of the totalingredients. Then the refined pulp is packaged in a baler 12.

Referring to FIG. 2, there is a block diagram of an embodiment of a dryprocess for making “dry” pulp in accordance with the present invention.This dry process is the same as the wet process except after thehorizontal water vacuum filter 9. After that filter, the pulp goesthrough a press 13 which removes more water until the pulp has aconcentration of water of no more than 20 wt % of the total ingredients.Then the pulp goes through a fluffer 14 to fluff the pulp and then arotor 15 to remove more water. Then, like the wet process, the pulp ispassed through a dryer 11 and packaged in a baler 12.

II. Second Embodiment of the Inventive Process

In a second embodiment, the process for making the acrylic fiber andpara-aramid pulp is the same as the first embodiment of the processdescribed above with the following differences.

Prior to combining all ingredients together, either the acrylic fiber orthe para-aramid fiber, or both the acrylic fiber and the para-aramidfiber, may need to be shortened. This is done by combining water withthe fiber ingredient. Then the water and fiber are mixed to form a firstsuspension and processed through a first disc refiner to shorten thefiber. The disc refiner cuts the fiber to an average length of no morethan 10 cm. The disc refiner will also partially fibrillate andpartially masticate the fiber. The other fiber, that was not previouslyadded, can be shortened this way too forming a second processedsuspension. Then the other fiber (or the second suspension, if processedin water) is combined with the first suspension.

More water is added before or after, or when, other ingredients areadded, if necessary, to increase the water concentration to 95-99 wt %of the total ingredients. After all ingredients are combined, they canbe mixed, if necessary, to achieve a substantially uniform slurry.

The ingredients in the slurry are then co-refined together, i.e.,simultaneously. This refining step includes fibrillating, cutting andmasticating solids in the suspension such that all or substantially allof the acrylic and para-aramid fiber is converted to irregularly shapedfibrillated fibrous structures. This refining step also disperses allsolids such that the refined slurry is substantially uniform. Then wateris removed as in the first embodiment of the process. Both processesproduce the same or substantially the same acrylic and para-aramid pulp.

The Inventive Pulp

The resulting product produced by the process of this invention is anacrylic and para-aramid pulp for use as reinforcement material inproducts. The pulp comprises (a) irregularly shaped, acrylic fibrousstructures, (b) irregularly shaped, para-aramid fibrous structures, (c)optionally substantially fibril-free, granular, para-aramid particles,(d) optionally other minor additives, and (e) water.

The concentration of the separate ingredient components in the pulpcorrespond, of course, to the concentrations described beforehand of thecorresponding ingredients used in making the pulp.

The irregularly shaped, acrylic and para-aramid fibrillated fibrousstructures have stalks and fibrils. The acrylic fibrils and/or stalksare substantially entangled with the para-aramid fibrils and/or stalks.The fibrils are important and act as hooks or fasteners or tentacleswhich adhere to and hold adjacent particles in the pulp and finalproduct thereby providing integrity to the final product.

The acrylic and para-aramid fibrillated fibrous structures preferablyhave an average maximum dimension of no more than 5 mm, more preferably0.1 to 4 mm, and most preferably 0.1 to 3 mm. The acrylic andpara-aramid fibrillated fibrous structures preferably have alength-weighted average of no more than 1.3 mm, more preferably 0.7 to1.2 mm, and most preferably 0.75 to 1.1 mm.

If para-aramid particles are included in the pulp, the acrylic andpara-aramid fibrous structures also additionally contact and are wrappedpartially around at least some of these rounder, substantiallyfibril-free, para-aramid particles. These para-aramid particle alsopreferably have a dimension of at least 50 microns, more preferably, 50to 100 microns, and most preferably 50 to 75 microns. Fibrils on andalong the acrylic and para-aramid fibrous structures can contact andform a partial cocoon around the rounder, substantially fibril-free,para-aramid particles

The acrylic and para-aramid pulp is without substantial aggregates orconglomerates of the same material. Further, the pulp has a CanadianStandard Freeness (CSF) as measured per TAPPI test T 227 om-92, which isa measure of its drainage characteristics, of 100 to 700 ml, andpreferably 250 to 450 ml.

Surface area of pulp is a measure of the degree of fibrillation andinfluences the porosity of the product made from the pulp. Preferably,the surface area of pulp of this invention is 7 to 11 square meters pergram.

FIG. 4 is an image of a photomicrograph of acrylic and para-aramid pulpmade according to the process of the present invention.

It is believed that aramid particles and fibrous structures, dispersedsubstantially homogeneously throughout the reinforcement material, andthe friction and sealing materials, provide, by virtue of the hightemperature characteristics of the para-aramid polymers and thefibrillation propensity of the para-aramid fibers, many sites ofreinforcement and increased wear resistance. When co-refined, theblending of the acrylic and para-aramid materials is so intimate that ina friction or sealing material there is always some para-aramid fibrousstructures close to the acrylic structures, so the stresses and abrasionof service are always shared.

Sealing Material

The invention is further directed to sealing material and processes formaking the sealing materials. Sealing materials are used in or as abarrier to prevent the discharge of fluids and/or gases and used toprevent the entrance of contaminants where two items are joinedtogether. An illustrative use for sealing material is in gaskets. Thesealing material comprises a binder; optionally at least one filler; anda fibrous reinforcement material comprising the acrylic and para-aramidpulp of this invention. Suitable binders include nitrile rubber,butadiene rubber, neoprene, styrene-butadiene rubber, nitrile-butadienerubber, and mixtures thereof. The binder can be added with all otherstarting materials. The binder is typically added in the first step ofthe gasket production process, in which the dry ingredients are mixedtogether. Other ingredients optionally include uncured rubber particlesand a rubber solvent, or a solution of rubber in solvent, to cause thebinder to coat surfaces of the fillers and pulp. Suitable fillersinclude barium sulfate, clays, talc, and mixtures thereof.

Suitable processes for making sealing materials are, for example, abeater-add process or wet process where the gasket is made from a slurryof materials, or by what is called a calendering or dry process wherethe ingredients are combined in an elastomeric or rubber solution.

Friction Material

The pulp of the present invention can be used as a reinforcementmaterial in friction materials. By “friction materials” is meantmaterials used for their frictional characteristics such as coefficientof friction to stop or transfer energy of motion, stability at hightemperatures, wear resistance, noise and vibration damping properties,etc. Illustrative uses for friction materials include brake pads, brakeblocks, dry clutch facings, clutch face segments, brake padbacking/insulating layers, automatic transmission papers, and frictionpapers.

In view of this new use, the invention is further directed to frictionmaterial and processes for making the friction material. Specifically,the friction material comprises a friction modifier; optionally at leastone filler; a binder; and a fibrous reinforcement material comprisingthe acrylic and para-aramid pulp of this invention. Suitable frictionmodifiers are metal powders such as iron, copper and zinc; abrasivessuch as oxides of magnesium and aluminum; lubricants, such as syntheticand natural graphites, and sulfides of molybdenum and zirconium; andorganic friction modifiers such as synthetic rubbers and cashew nutshell resin particles. Suitable binders are thermosetting resins such asphenolic resins (i.e., straight (100%) phenolic resin and variousphenolic resins modified with rubber or epoxy), melamine resins, epoxyresins and polyimide resins, and mixtures thereof. Suitable fillersinclude barite, calcium carbonate, wollastonite, talc, various clays,and mixtures thereof.

The actual steps for making the friction material can vary, depending onthe type of friction material desired. For example, methods for makingmolded friction parts generally involve combining the desiredingredients in a mold, curing the part, and shaping, heat treating andgrinding the part if desired. Automotive transmission and frictionpapers generally can be made by combining the desired ingredients in aslurry and making a paper on a paper machine using conventional papermaking processes.

Test Methods

The following test methods were used in the following Examples.

Canadian Standard Freeness (CSF) is a well-known measure of the facilityfor water to drain from a slurry or dispersion of particles. Freeness isdetermined by TAPPI test T227. Data obtained from conduct of that testare expressed as Canadian Freeness Numbers, which represent themilliliters of water which drain from an aqueous slurry under specifiedconditions. A large number indicates a high freeness and a high tendencyfor water to drain. A low number indicates a tendency for the dispersionto drain slowly. The freeness is inversely related to the degree offibrillation of the pulp, since greater numbers of fibrils reduce therate at which water drains through a forming paper mat.

Length-weighted average is measured using a “FiberExpert” tabletopanalyzer (also now known as “PulpExpertFS”, available from MetsoAutomation of Helsinki, Finland). This analyzer takes photographicimages of the pulp with a digital CCD camera as the pulp slurry flowsthrough the analyzer and then an integrated computer analyzes the fibersin these images and calculates their length-weighted average.

Temperature: All temperatures are measured in degrees Celsius (° C.).

Denier is measured according to ASTM D 1577 and is the linear density ofa fiber as expressed as weight in grams of 9000 meters of fiber.

The denier is measured on a Vibroscope from Textechno of Munich,Germany. Denier times (10/9) is equal to decitex (dtex).

EXAMPLES

This invention will now be illustrated by the following specificexamples. All parts and percentages are by weight unless otherwiseindicated. Examples prepared according to the process or processes ofthe current invention are indicated by numerical values.

Example 1

In this example of the invention, the pulp of this invention wasproduced from a feedstock of para-aramid fiber and acrylic staple.Acrylic staple having a cut length of 2 inches and having a filamentlinear density of 3 dpf (3.3 dtex per filament) was obtained fromSolutia, Inc., with offices in St. Louis, Mo. Para-aramid fiber in theform of commercially available KEVLAR® brand floc, Style 1F178, having a¼″ cut length, was obtained from E. I. de Pont de Nemours and Companywith offices in Wilmington, Del.

Acrylic staple and water together were fed directly into aSprout-Waldron 12″ Single Disc Refiner using a 10 mil plate gap settingand pre-pulped to reach an acceptable processing length in the range of13 mm.

The pre-pulped acrylic fiber and the cut para-aramid fiber plus waterwere then combined into a highly agitated mixing tank at a solidsconcentration of 50 wt % para-aramid fiber and 50 wt % acrylic stapleand mixed to form a uniform, pumpable slurry of about 2-3 wt % of thetotal ingredients concentration. The slurry was then recirculated andco-refined through a Sprout-Waldron 12″ Single Disc Refiner.

The refiner simultaneously:

-   -   (1) fibrillated, cut, and masticated both the para-aramid fiber        and the acrylic staple to irregularly shaped fibrous structures        having stalks and fibrils.    -   (2) dispersed all solids such that the refined slurry was        substantially uniform, substantially uniform being as previously        defined.

This refined slurry was then filtered using a filter bag and wasdewatered through pressing and placed in large ZIPLOC® type storagebags. The fibrous structures had an average maximum dimension of no morethan 5 mm and a length-weighted average of no more than 1.3 mm, asmeasured by FiberExpert®.

Example 2

This example illustrates another method by which a co-refined pulp canbe made from a feedstock of para-aramid fiber and acrylic fiber. Acrylicstaple, having a cut length of 2 inches and having a filament lineardensity of 3 dpf (3.3 dtex per filament) available from Solutia, Inc.,is cut with a guillotine cutter two to three times at right angles inorder to produce a random-length fiber with most fibers shorter than ¾inch (1.91 cm) and averaging about ½ inch (1.27 cm) long.

Para-aramid fiber in the form of commercially available KEVLAR® brandmultifilament yarn, available from E. I. de Pont de Nemours and Companyon bobbins, is prepared by cutting the para-aramid yarn to a nominal ½inch (1.27 cm) cut length on a Lummus Cutter (available from LummusIndustries with offices in Columbus, Ga.). Other KEVLAR® brandpara-aramid fiber, which initially is not on bobbins and is of multiplelong lengths, is cut by a guillotine cutter two to three times at rightangles in order to produce a random-length fiber with most fibersshorter than ¾ inch (1.91 cm) and averaging about ½ inch (1.27 cm) long.

The two ingredients prepared as described above plus water are thencombined into a highly agitated mixing tank called a hydrapulper at asolids concentration of 50 wt % para-aramid fiber and 50 wt % acrylicfiber and mixed to form a substantially uniform, pumpable slurry havinga total solids concentration of about 2-3 wt % of the total ingredients.The slurry is pumped through a series of three refiners, as described inU.S. Pat. No. 4,472,241. The refiners simultaneously:

-   -   (1) fibrillate, cut, and masticate the acrylic fiber and the        para-aramid fiber into irregularly shaped fibrous structures        having stalks and fibrils; and    -   (2) disperse all solids such that the refined slurry was        substantially uniform with substantially uniform as previously        defined.

This refined slurry is then dewatered using a horizontal filter anddried in an oven to a desired moisture content of 50 total wt % for wetpulp. The wet pulp is then packaged into bales by a baler. When measuredby FiberExpert®, all of the ingredients in the pulp have alength-weighted average of no more than 1.3 mm.

Example 3

This example illustrates further process steps and another embodiment ofthe pulp of this invention. The procedure of Example 2 is followed.However, after the pulp is dewatered on the horizontal filter, the pulpis pressed in a mechanical press to further remove water; and the pulpis then fluffed using a Fluffer (available from Bepex Corporation withoffices at Santa Rosa, Calif.) to better separate the pressed wet pulp.The fluffed wet pulp is then dried in an oven to approximately 8 totalwt % moisture and is then further processed in an ultrarotor (model IIIAavailable from Altenburger Machinen Jackering GmbH with offices inVoisterhauser, Germany) such as is disclosed in U.S. Pat. No. 5,084,136to further fluff and disperse the dried pulp. The dried pulp is thenpackaged into bales. When measured by FiberExpert®, all of theingredients in the pulp have a length-weighted average of no more than1.3 mm.

Example 4

This example illustrates another embodiment of the pulp of thisinvention. The process of Example 2 is followed with the exception thatone third by weight of the para-aramid fiber is replaced by para-aramidparticles. The para-aramid resin particles are prepared by reactingpara-phenylenediamine and teraphthaloyl chloride continuously in a screwextruder as is generally disclosed in U.S. Pat. No. 3,884,881, but usingN, methyl pyrollidone/calcium chloride as the solvent, producing acrumb-like polymer that precipitates from the solvent. The solvent isextracted, and the polymer crumb washed and dried to a particulatepowder of mixed particle size. The para-aramid resin particles are thentreated substantially the same as the para-aramid fiber is treated inExample 2. However, the refiner not only refines the fibers but alsocuts and/or masticates the para-aramid particles into rounder,substantially fibril-free particles. After dewatering, some of theresulting pulp having a moisture content of 50 total wt percent is thenpackaged into bales. The remainder of the resulting pulp is furtherpressed to a moisture content of approximately 8 total wt percent andthen fluffed, dispersed, and packaged as in Example 3. When measured byFiberExpert®, all of the ingredients in the pulp have a length-weightedaverage of no more than 1.3 mm.

Example 5

Disc brake pads incorporating the pulp of this invention were made inthe following manner. Approximately 20 kilograms of anon-asbestos-containing base compound powder comprising a mixture of 7wt % cashew nut shell resin, 17 wt % inorganic fillers, 21 wt %graphite, coke and lubricants, 18 wt % inorganic abrasives, and 16 wt %soft metals was mixed together for 10 to 20 minutes in a 50-literLittleford mixer. The mixer had two high-speed choppers with blades ofthe “stars and bars” configuration and a slower rotating plough.

5 kilograms of the well-blended base compound powder was then combinedwith the pulp of this invention (a co-refined pulp being 50 wt %para-aramid and 50 wt % acrylic fiber) in an amount of 3.8 wt %, basedon the combined weight of the compound powder and the pulp. The pulp wasthen dispersed in the base compound powder by mixing for an additional 5to 10 minutes. Once mixed, the resulting brake pad composition had anormal visual appearance with the fiber well dispersed in and completelycoated with the base compound powders, with essentially no detectableballing up of the pulp or segregation of any constituents.

The brake pad composition was then poured into a single-cavity steelmold for a front disc brake pad and cold pressed to a standard thicknessof about ⅝ inch (16 mm) then removed from the mold to form a pre-formedbrake pad having an approximate weight of 200 grams. The pre-form had noexcessive spring-back or swelling, and was robust enough to endurenormal handling without damage. Twelve replicate pre-forms were made.The pre-forms were then placed in two multi-cavity molds, placed in acommercial press, and press-cured (the binder phenolic cross-linking andreacting) at 300° F. (149° C.) for about 15 minutes, with periodicpressure release to allow phenolic reaction gases to escape, followed bylightly constrained oven curing at 340° F. (171° C.) for 4 hours tocomplete the phenolic binder crosslinking. The cured, molded pad wasthen ground to the desired thickness of about half an inch (13 mm). Whencompared visually with a commercial brake pad containing an equivalentamount of all para-aramid pulp or acrylic pulp, the test pad wasindistinguishable and had good compound flow into the backing plateholes and no edge chipping.

A sample of the brake pad incorporating the pulp of this invention wasthen tested to determine its frictional performance. Coupons, typicallyone inch by one inch and about 3/16 inch (5 mm) thick, from test padswere assessed on the Chase Machine available from Link Engineering,Detroit, Mich., using test protocol Society of Automotive Engineers(SAE) J661 to determine the hot and cold friction coefficient duringconstant pressure and controlled temperature drag tests against a heatedsteel drum. The sample was periodically measured for wear (thicknessloss). This was repeated with two more test samples cut from otherreplicate pads. The samples of the brake pad incorporating the pulp ofthis invention exhibited hot and cold friction performance substantiallyequivalent to commercially available pads containing a substantiallyequivalent amount of all para-aramid pulp. The test further indicatedthe pad-to-pad uniformity and an average friction rating was alsosubstantially equivalent.

The pad was then tested for friction and wear under various brakingconditions using a dynamometer (single piston dynamometer with a rollingradius of 289.0 mm at Link Testing Laboratories, Inc., in Detroit,Mich.) using test protocol J2681 (ISO-SWG4). This test was comprised ofseventeen scenarios of from 5 to 200 brake applications each, andmeasured coefficient of friction as a function of applied brakepressure, temperature, braking speed and deceleration rate. This testalso had two high-temperature fade sections, during which the brake padwas subjected to increasingly high initial temperatures during constantdeceleration, and reached temperatures exceeding 600° C. Wear wasmeasured as the reduction in thickness and weight of the pad at the endof the test (608 brake applications.) Results for the pads made with thecompound of this example showed very little fade and what fade there wasrecovered well (where fade is defined as the loss of friction at thehighest temperature brake applications), acceptable coefficient offriction of 0.25 to 0.4 in non-fade sections, absence of pad surfacecracking, and acceptable wear rates for both the pad and the rotor.

Example 6

This example illustrates how the pulp of this invention can beincorporated into a beater-add gasket for sealing applications. Water,rubber, latex, fillers, chemicals, and the pulp of this invention arecombined in desired amounts to form a slurry. On a circulating wiresieve (such as a paper machine screen or wire), the slurry is largelydrained of its water content, is dried in a heating tunnel, and isvulcanized on heated calender rolls to form a material having a maximumthickness of around 2.0 mm. This material is compressed in a hydraulicpress or two-roll calender, which increases the density and improvessealability.

Such beater-add gasket materials generally do not have as goodsealability as equivalent compressed-fiber materials and are best suitedfor moderate-pressure high-temperature applications. Beater-add gasketsfind applicability in the making of auxiliary engine gaskets or, afterfurther processing, cylinder head gaskets. For this purpose, thesemi-finished product is laminated onto both sides of a spiked metalsheet and is physically fixed in place by the spikes.

Example 7

This example illustrates how the pulp of this invention can beincorporated into a gasket made by a calendering process. The sameingredients as in Example 6, minus the water, are thoroughly dry mixedtogether and are then blended with a rubber solution prepared using anappropriate solvent.

After mixing, the compound is then generally conveyed batchwise to aroll calender. The calender consists of a small roll that is cooled anda large roll that is heated. The compound is fed and drawn into thecalender nip by the rotary movement of the two rolls. The compound willadhere and wrap itself around the hot lower roll in layers generallyabout 0.02 mm thick, depending on the pressure, to form a gasketingmaterial made from the built-up compound layers. In so doing, thesolvent evaporates and vulcanization of the elastomer commences.

Once the desired gasketing material thickness is reached, the rolls arestopped and the gasketing material is cut from the hot roll and cutand/or punched to the desired size. No additional pressing or heating isrequired, and the material is ready to perform as a gasket. In thismanner gaskets up to about 7 mm thick can be manufactured. However, mostgaskets made in this manner are much thinner, normally being about 3 mmor less in thickness.

1. A process for making an acrylic and para-aramid pulp for use as reinforcement material, comprising: (a) combining pulp ingredients including: (1) acrylic fiber comprising acrylonitrile units which are at least 85 wt % of the total acrylic fiber, the fiber being 10 to 90 wt % of the total solids in the ingredients, and having an average length of no more than 10 cm; (2) para-aramid fiber being 10 to 90 wt % of the total solids in the ingredients, and having an average length of no more than 10 cm; and (3) water being 95 to 99 wt % of the total ingredients; (b) mixing the ingredients to a substantially uniform slurry; (c) co-refining the slurry by simultaneously: (1) fibrillating, cutting and masticating the acrylic fiber and the para-aramid fiber to irregularly shaped fibrillated fibrous structures with stalks and fibrils; and (2) dispersing all solids such that the refined slurry is substantially uniform; and (d) removing water from the refined slurry to no more than 60 total wt % water, thereby producing an acrylic and para-aramid pulp with the acrylic and the para-aramid fibrous structures having an average maximum dimension of no more than 5 mm, a length-weighted average of no more than 1.3 mm, and the acrylic fibrils and/or stalks are substantially entangled with the para-aramid fibrils and/or stalks.
 2. The process of claim 1, wherein the acrylic fiber having a linear density of no more than 10 dtex; and the para-aramid fiber having a linear density of no more than 2.5 dtex.
 3. The process of claim 1, wherein the pulp being without substantial aggregates of the same material.
 4. The process of claim 1, wherein the ingredients further comprise substantially or completely fibril-free, granular, para-aramid particles being no more than 50 wt % of the total solids in the ingredients, and having an average maximum dimension of 50 to 2000 microns, and in the refining step, masticating at least some of the para-aramid particles into smaller, rounder, substantially fibril-free, particles, whereby in the produced acrylic and para-aramid pulp, the acrylic and para-aramid fibrous structures contact and are wrapped partially around at least some of the rounder, substantially fibril-free, para-aramid particles.
 5. The process of claim 1, wherein in the combining step, the acrylic fiber comprises 25 to 60 wt % of the total solids.
 6. The process of claim 1, wherein in the combing step, the para-aramid fiber comprises 40 to 75 wt % of the total solids.
 7. The process of claim 1, wherein after the removing step, the water being 4 to 60 wt % of the entire pulp, and the pulp having a Canadian Standard Freeness (CSF) of 100 to 700 ml.
 8. The process of claim 1, wherein the refining step comprises passing the mixed slurry through a series of disc refiners.
 9. A process for making an acrylic and para-aramid pulp for use as reinforcement material, comprising: (a) combining ingredients including water and a first fiber from the group consisting of: (1) acrylic fiber comprising acrylonitrile units which are at least 85 wt % of the total acrylic fiber, the fiber being 10 to 90 wt % of the total solids in the pulp; and (2) para-aramid fiber being 10 to 90 wt % of the total solids in the pulp; (b) mixing the combined ingredients to a substantially uniform suspension; (c) refining the suspension in a disc refiner thereby cutting the fiber to have an average length of no more than 10 cm, and fibrillating and masticating at least some of the fiber to irregularly shaped fibrillated fibrous structures; (d) combining ingredients including the refined suspension, the second fiber of the group of (a)(1 and 2) having an average length of no more than 10 cm, and water, if necessary, to increase the water concentration to 95-99 wt % of the total ingredients; (e) mixing the ingredients, if necessary, to form a substantially uniform suspension; (d) co-refining the mixed suspension by simultaneously: (1) fibrillating, cutting and masticating solids in the suspension such that all or substantially all of the acrylic and para-aramid fiber is converted to irregularly shaped fibrillated acrylic and para-aramid fibrous structures with stalks and fibrils; and (2) dispersing all solids such that the refined slurry is substantially uniform; and (h) removing water from the refined slurry to no more than 60 total wt % water, thereby producing an acrylic and para-aramid pulp with the acrylic and the para-aramid fibrous structures having an average maximum dimension of no more than 5 mm, a length-weighted average of no more than 1.3 mm, and the acrylic fibrils and/or stalks are substantially entangled with the para-aramid fibrils and/or stalks.
 10. The process of claim 9, wherein the ingredients further comprise: substantially or completely fibril-free, granular, para-aramid particles being no more than 50 wt % of the total solids in the ingredients, and having an average maximum length of 50 to 2000 microns; and in either the first or second refining step, masticating at least some of the para-aramid particles into smaller, rounder, substantially fibril-free, particles, whereby in the produced acrylic and para-aramid pulp, the acrylic and para-aramid fibrous structures contact and are wrapped partially around at least some of the rounder, substantially fibril-free, para-aramid particles.
 11. The process of claim 9, wherein after the removing step, the irregularly shaped acrylic fibrous structures being 25 to 60 wt % of the total solids.
 12. The process of claim 9, wherein after the removing step, the irregularly shaped, para-aramid, fibrous structures being 40 to 75 wt % of the total solids.
 13. The process of claim 9, wherein after the removing step, the water being 4 to 60 wt % of the entire pulp, and the pulp having a Canadian Standard Freeness (CSF) of 100 to 700 ml.
 14. An acrylic and para-aramid pulp for use as reinforcement material, comprising: (a) irregularly shaped, acrylic fibrous structures comprising acrylonitrile units which are at least 85 wt % of the total acrylic fibrous structures, the structures being 10 to 90 wt % of the total solids; (b) irregularly shaped, para-aramid fibrous structures being 10 to 90 wt % of the total solids; and (c) water being 4 to 60 wt % of the entire pulp, whereby the acrylic and the para-aramid fibrous structures having an average maximum dimension of no more than 5 mm, a length-weighted average of no more than 1.3 mm, and stalks and fibrils where the acrylic fibrils and/or stalks are substantially entangled with the para-aramid fibrils and/or stalks.
 15. The aramid pulp of claim 14, further comprising substantially or completely fibril-free, granular, para-aramid particles being no more than 50 wt % of the total solids.
 16. The pulp of claim 14, wherein the irregularly shaped, acrylic fibrous structures being 25 to 60 wt % of the total solids.
 17. The pulp of claim 14, wherein the irregularly shaped, para-aramid, fibrous structures being 40 to 75 wt % of the total solids.
 18. The pulp of claim 14, wherein the water being 4 to 60 wt % of the entire pulp, and the pulp having a Canadian Standard Freeness (CSF) of 100 to 700 ml.
 19. A friction material, comprising: a friction modifier; a binder; and a fibrous reinforcement material comprising the pulp of claim
 14. 20. The friction material of claim 19, wherein the friction modifier is selected from the group consisting of metal powders, abrasives, lubricants, organic friction modifiers, and mixtures thereof; and the binder is selected from the group consisting of thermosetting resins, melamine resins, epoxy resins and polyimide resins, and mixtures thereof.
 21. A sealing material, comprising: a binder; and a fibrous reinforcement material comprising the pulp of claim
 14. 22. The sealing material of claim 21, wherein the binder is selected from the group consisting of nitrile rubber, butadiene rubber, neoprene, styrene butadiene rubber, nitrile-butadiene rubber, and mixtures thereof. 